Non-invasive measuring of arterial blood flow characteristics such as arterial pulse rate, oxygen saturation level of hemoglobin in arterial blood, or other blood constituents is common in medical practice The measurements rely on noninvasive sensors that are placed against the patient's tissue at a location where the tissue is well perfused, for example, a fingertip, a toe, the earlobe, the nasal septum, the forehead, the ical cord, and the like. The sensors contain source for passing one or more more wavelengths of light into the perfused tissue and a light detector for detecting the amount of light passing through the tissue. The light source may be an incandescent lamo or one or more light emitting diodes ("LEDs") and the light detector may be a phototransistor, photodiode, or other photodetector. The light detector may be arranged adjacent the light source to detect the amount of light reflected from illuminated tissue such as the forehead or a fingertip, or at a location on the opposite side of the tissue to detect the amount of light passed through the illuminated tissue, such as a fingertip or the bridge of the nose.
The wavelengths used in the sensor are selected for the propensity of the light to be absorbed by the blood constituent being measured. The amount of detected light can then be related to the instantaneous quantity of the blood constituent present in the sample which relationship can be used to determine the amount of the blood constituent in accordance with known procedures, following, for example, Beer's Law. For oxygen saturation measurements, red and infrared wavelengths are typically used. As the oxygen level of the hemoglobin in the blood changes over time, the color of the hemoglobin changes and the amount of red and infrared light absorbed by the hemoglobin changes accordingly. The amount of light detected by the photodetector thus varies in accordance with the varying amount of light absorbed by the tissue and pulsatile arterial blood flow, and, from the amount of light absorbed, oxygen saturation of the blood can be determined. See, for example, the preferred method of calculating blood oxygen saturation (using a non-invasive sensor) described in U.S. patent application Ser. Nos. 718,525, filed April 1, 1985, and 742,720, filed June 7, 1985, both of which are copending and commonly assigned, the disclosure of which are incorporated herein by reference.
The light passing through the tissue is also affected by time invariant factors, including but not limited to skin, skin pigment, bone, nail, hair, other non-moving components, and the like. These factors absorb some of the light illuminating the tissue and the amount of absorption may vary from one patient to another. The intensity of the light from the light source may be ad]usted to compensate for such variations and to provide a light bright enough to be detected by the light detector. The detected signal thus contains substantially only the time-varying portion of the light representing the arterial pulsatile flow and the varying amount of blood constituent, for example, oxygenated hemoglobin. The amount of detected light can be correlated to provide the measure of the quantity of the oxygen saturation present in the blood. The technique also can be used for measuring other blood constituents that absorb light of selected frequencies.
Instruments based on this and other similar optical transmission measuring principles have been designed, developed, and are commercially available. Typically, they use two or more wavelengths to measure oxygen saturation and pulse rate. The Nellcor N-100 Oximeter, manufactured by Nellcor Incorporated, Hayward, Calif., is one such instrument. These instruments typically control the transmission of light by the sensor in a pulsed fashion at an appropriate rate, detect the light transmitted, and use sophisticated electronic signal processing devices that process the detected light related information and perform the desired calculations. See U.S. patent application Ser. Nos. 718,525 filed April 1, 1985 and 742,720, filed June 7, 1985.
Known sensors for use with pulse detection (plethysmography) and oxygen detection (oximetry) include reflective mode sensors where the light source and the photodetector are mounted in a rigid surface that is applied to and secured against the tissue by means of a strap about the appendage or the head. See U.S. Pat. Nos. 3,167,658, 4,380,240 and 4,321,930. Alternate forms of reflective mode sensors may include flexible structures that rely on adhesive tape to secure the sensor to the tissue, such as is referred to in U.S. Pat. No. 4,510,938. One reflective mode sensor design provides for a somewhat rigid, but still flexible, conically sectioned sleeve structure that is slipped over a finger until wedged against it, as referred to in U.S. Pat. No. 4,425,921.
Another known form of sensor is a transmissive mode sensor of a clip or clothes-pin type arrangement where one leg has the light source mounted in its rigid flat surface and the opposing leg has the light detector mounted in its rigid flat surface. The two legs are clamped against and on opposite sides of the tissue or an appendage such as a finger or an earlobe. See U.S. Pat. Nos. 3,152,587 and 3,810,460. These clamps may have either a screw arrangement or a spring to secure the sensor legs to the tissue. Other transmissive sensor designs have the light source and light detector mounted in a flexible structure having an adhesive surface for taping the sensor securely around the tissue such as a finger, the foot, or the bridge of the nose. See U.S. patent applications Ser. No. 539,865 filed Oct. 7, 1983 and Ser. No. 493,442 filed May 11, 1983, both of which are copending and commonly assigned. Combined transmissive and reflective sensors designed to selectively detect blood flow from capillary blood flow and palmer finger arteries, as referred to in U.S. Pat. No. 4,013,067, are also known.
One problem with sensors having rigid surfaces containing the light source and light detector is that the surfaces do not conform to the tissue. Rather, the tissue must conform to the sensor in order to provide the close coupling needed to exclude as much ambient light as possible and retain as much light generated by the light source as possible. Close coupling provides for a better signal to noise ratio, a better quality signal and, therefore, more reliable and accurate measurements.
A second problem with rigid housings is that they have a significant mass which can easily become dislodged accidentally, inadvertantly, or intentionally, in the course of measuring. Such movement can result in a less than desirable light path, for example, through not well perfused tissue or, in part, not through any tissue. Movement also can change the length or direction of the light path which results in a change in detected light intensity and can affect the calibration of the instrument. These changes can degrade the quality and reliability of the signal and thus the accuracy of the measurements.
To prevent movement, the sensor must be securely held against the tissue by some force. This force consequently exerts pressure on the tissue, which has a tendency to compress the tissue adjacent the sensor, restrict blood flow into the tissue adjacent the sensor, and expel venous blood from the tissue. Such forces can cause reduced arterial or venous blood flow. Further, the hard surfaces may cause localized pressure points which result from the surfaces capturing an area of tissue, pinching it, and thereby causing injury. In addition, rigid sensors can leave compression marks on the tissue which indicates that venous blood has been forced out of the localized area and the blood flow thereto has been altered or minimized. The effect of these results are undesired and not in accordance with generally accepted medical practices for non-invasive measuring techniques and patient well-being. These effects may also operate on normally invariant factors and are work related (pressure, time, distance) so that the quality of the signal may gradually degrade and result in false measurements.
To minimize the impact of the constant pressure exerted by these rigid and massive sensors, the devices must be adjusted or repositioned with regularity. This does not permit long periods of unattended or uninterrupted measurements.
Yet another problem with rigid sensors is that of motion artifact Motion artifacts are detected pulses that come from muscular or skeletal movement of the tissue or relative movement between the sensor and the tissue. These motions can affect normal pulsatile blood flow causing artificial arterial pulses or change the distance between the light source and the photodetector causing rapid changes in light intensity, each of which can result in erroneous measurements. Motion artifact produces spurious pulses that do not result from true arterial pulsatile flow based upon the heartbeat.
The sensors mounted in flexible structures for contacting the tissue do have conformance to the tissue and, if mounted properly, may not exert any significant pressure against the tissue. However, the pressure is dependent upon the structure's degree of flexibility and how it corresponds to the curvature and flexibility of the tissue. The pressure also depends upon the means for securing the sensor to the tissue, typically a web that is secured about the head or other limb, which is not uniformly controlled or regulated.
Adherent sensors have the advantage that they do not move relative to the tissue, thus reducing the cause of at least one source of measurement error, sensor movement. Adherent flexible sensors have solved some of the problems confronted by the rigid sensors, i.e., conformance, no relative motion, but they have a very short usable lifetime and typically, are restricted to one application. Furthermore, the adhesive surface can lose its adherent quality when moistened for example by blood, body fluids, or perspiration, and does not retain its tackiness beyond a few applications. In addition, adhesives cannot be readily cleaned and sterilized or replaced with any degree of economic efficiency.
It is therefore an object of this invention to provide a durable sensor having a soft deformable pad for contacting and conforming to a variety of different sized tissues for use in connection with detection of selected blood constituents, wherein the sensor may be cleaned and used repeatedly, and will provide accurate, un-interrupted measurements over long periods of time.
Another object of this invention is to provide for a pulse oximeter sensor that is a clothes-pin type clamp for use in detecting oxygen saturation levels from the finger of a patient and exerts enough force to remain secured to the finger even when moved, but not so much force tha& would cause localized pressure points, patient discomfort and compression marks, indicating the blood flow to the region has been altered, or to compress the tissues to such an extent that it affects the accuracy of the measurements over time.